Discharge-lamp starting circuits producing high voltage pulses



May 5, 1970 K. w. OGDEN ETAL 3,510,727 I DISCHARGE-LAMP STARTING CIRCUITS PRODUCING HIGH VOLTAGE PULSES Filed Nov. 29.1966

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KENNETH WHITTER OGDEN DONALD FREDERICK CHAPMAN INVENTORS ATTORNEY United States Patent 3,510,727 DISCHARGE-LAMP STARTING CIRCUITS PRODUCING HIGH VOLTAGE PULSES Kenneth Whittier Ogden and Donald Frederick Chapman, London, England, assignors to British Lighting Industries Limited, London, England Filed Nov. 29, 1966, Ser. No. 597,825 Claims priority, application Great Britain, Dec. 14, 1965, 53,106/ 65 Int. Cl. Gf 1/08; H05b 41/231, 41/232 US. Cl. 315194 3 Claims ABSTRACT OF THE DISCLOSURE An electric discharge lamp starting circuit having a pair of semiconductor A.C. switches of opposite polarities in shunt to each other with a resistance a condenser and one part of a choke coil in series with the switch combination, the other part of the choke coil being in series with the lamp to be started, and a resistor across the condenser to regulate the phase of the starting voltage pulses produced.

The present invention relates to starting circuits for discharge lamps which produce high voltage pulses to initiate the discharge through the lamp.

Circuits of this kind are already known in which a pair of relay contacts in parallel with the lamp are are closed momentarily to cause a surge of current which passes through part of the normal choke in series with the lamp and induces across the choke a short voltage pulse of large amplitude. Such circuits are used with mercury discharge lamps which may require a starting voltage of as much as 2 kv.

In accordance with the present invention there is provided a starting circuit for a discharge lamp having a choke coil at least a major part of which is connected in series with the lamp terminals across the AC. supply terminals and a circuit connected in parallel with the lamp terminals across the supply terminals and including a minor part of the choke coil, a capacitor and resistor in parallel, and a semiconductor A.C. switching apparatus having a break-over voltage such that on each halfcycle of the AC. supply it remains in a nonconducting condition until the voltage of the supply approaches its peak value and thereafter allows a surge of current to pass through the said minor part of the choke coil and induce a high voltage pulse across the rest of the choke coil.

The high voltage pulses applied to the lamp may be in phase or antiphase with the initial peaks of the normal operating voltage curve of the lamp. The initial peaks of the normal operating voltage curve are those which occur before the discharge is restarted in each half-cycle of operation and which appear as projections or spikes on a waveform of approximately square wave shape.

In a preferred form of the circuit the lamp is connected to one end of the choke coil, the said major part extends between the end connected to the lamp and a connection to a supply terminal and the said minor part extends from the supply terminal connection to the other end of the choke coil. This ensures that the high voltage pulses are in the same sense as the supply voltage.

The magnitude of the voltage pulses can be controlled by varying the turns ratio between the two parts of the choke coil and by inserting resistance in the circuit connected in parallel with the lamp. The phase of the pulses can be adjusted by changing the relative values of the circuit components, particularly the resistor in parallel with the capacitor.

The semiconductor A.C. switching apparatus may be a single device of the type variously known as a Symmistor, Triac, or controlled A.C. switch which has the required characteristic of a break-over voltage for both directions of current flow or may be made up of two devices connected in parallel in opposite senses, each device having a high resistance in one direction and a break-over voltage in the other direction. For this purpose it is possible to use Shockley diodes or thyristors, the control electrodes of the latter being left floating. A Syrnmistor is a five element thyristor, and is defined in a paper entitled Thyristor, Design and Performance, appearing in the Electrical Times of Nov. 25, 1965, page 817.

The invention will now be described in more detail with the aid of examples illustrated in the accompanying drawings, FIGS. 1 and 2 of which are circuit diagrams of two different starting circuits for a mercury discharge lamp.

In FIG. 1 the lamp 10 is of the Metalarc type in which the filling incorporates iodine and thorium or other metals to change the character of the emitted light. Such lamps are described in US. applications Ser. Nos. 209,974 of July 16, 1962, 230,044 of Oct. 16, 1962 and 239,272 of Nov. 21, 1962. The lamp operates at 3.52 amps, 400 watts and is to be energised from 240 volt 50 cycle A.C. mains connected to the live (L) and neutral (N) terminals of the starting circuit. One electrode of the lamp 10 is connected directly to the neutral terminal while the other electrode is connected to one end of a conventional choke coil 11. The live terminal is connected to a choke coil 11 at a point 12 which divides the coil in the ratio of 4 to l, the total impedance of the choke coil at 50 cycles being 60 ohms, and the impedance of the minor part 4 ohms. The other end of the coil is connected to the neutral terminal through a parallel combination of a capacitor 13 and resistor 14, an inverse parallel combination of two Shockley diodes 15 and 16, and a resistor 17. Typical values of the circuit elements are shown in the drawing.

The diodes 15 and 16 are connected in opposite senses and each has a high resistance in one direction and a break-over voltage in the other direction so that as the peak of each half-cycle of the mains supply voltage is approached one or the other of the diodes suddenly allows a large current to flow to charge the capacitor 13, the current being limited only by the resistor 17, and the impedance of the minor part of the choke coil. The flow of the current through the smaller part of the choke coil induces a considerably higher voltage, of the order of 2 kv., across the larger part of the coil. Thus in each half cycle a short high voltage starting pulse is applied to the lamp. When the discharge in the lamp has been established the starting circuit continues to operate but the high voltage pulses are short-circuited through the lamp, which operates at a much lower voltage between its electrodes. The voltage across the lamp during normal operation is usually out of phase with the supply voltage because of the action of the choke coil 11 and the starting voltage pulses should be arranged to be in phase with the initial peaks of the operating voltage. This can be done by changing the value of the resistor 14, or the value of the capacitor 13.

An alternative circuit is shown in FIG. 2 in which the high voltage pulses are produced in antiphase to the supply voltage. We have found that this arrangement will also start Metalarc type lamps satisfactorily. In FIG. 2 one electrode of the lamp 10 is connected directly to the neutral terminal (N) while the other electrode is connected to one end of a choke coil 11. The live terminal (L) is connected to the other end of the choke coil. The tapping point 12 on the choke is connected to the neutral terminal through a parallel combination of capacitor 13 and resistor 14, an inverse parallel combination of two Shockley diodes, and a resistor 17. The large flow of inrush current to charge the capacitor 13, which occurs when one or other of the diodes breaks-over, flows through the smaller part of the choke coil between tapping 12 and the live terminal (L). This induces a high pulse voltage of short duration across the whole of the choke coil. The pulse voltage appears across the lamp terminals and is in opposition to the A.C. supply voltage appearing at the lamp terminals during starting. The high voltage pulses are produced repetitively during each half cycle and their phase, relative to the supply voltage, can be adjusted by changing the value of the resistor 14 or the value of the capacitor 13.

When the discharge in the lamp has been established the voltage appearing across diodes 15 and 16 is reduced and may be below their break-over voltage. The starting circuit can therefore be made inoperative whilst the lamp is running.

We claim:

.1. A discharge lamp-starting circuit comprising A.C. supply terminals, a choke coil, means connecting at least a major part of the choke coil in series with the lamp across the supply terminals, and a pulse-generating circuit connected in parallel with the lamp across the supply terminals, said pulse-generating circuit comprising a minor part of the choke coil, a capacitor, a resistor to shift the phase thereof, said capacitor being in parallel with said resistor, and a semiconductor A.C. switching apparatus having a break-over voltage such that on each half-cycle of the A.C. supply it remains in a nonconducting condition until the voltage of the supply approaches its peak value and thereafter allows a surge of current to pass through the said minor part of the choke coil and induce a high voltage pulse across the rest of the choke coil.

2. A circuit as claimed in claim 1 in which the lamp is connected to one end of the choke coil, the said major part extends between the end connected to the lamp and a connection to a supply terminal and the said minor part extends from the supply terminal connection to the other end of the choke coil.

3. A circuit as claimed in claim 1 in which the semiconductor A.C. switching apparatus comprises two devices connected in parallel in opposite senses, each device having a high resistance in one direction and a break-over voltage in the opposite direction.

References Cited UNITED STATES PATENTS 2,614,243 10/ 1952 Clark 315-245 X 3,037,147 5/ 1962 Genuit et al 315205 X 3,259,797 7/1966 Heine et al 315-194X 3,274,580 9/ 1966 Thomson 315245 X 3,235,769 2/1966 Wattenbach 315-176 JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner US. Cl. X.R. 

